Discussion Document: Energy Storage and Power Electronics on the Low Voltage Distribution Network
Problem statement, hypothesis and test deployment programme
1.Introduction
New ThamesValley Vision
The New Thames Valley Vision (NTVV) aims to demonstrate that understanding, anticipating and supporting changes in consumer behaviour will help DNOs to develop an efficient network for the low carbon economy. This £30 million project is part of a £500 million programme funded by the Low Carbon Network Fund (LCNF) run by Ofgem, the UK energy regulator.
Learning Outcomes
The project explores five central learning outcomes:
1Understanding - What do we need to know about customer behaviour in order to optimise network investment?
2Anticipating - How can improved modelling enhance network operational, planning and investment management systems?
3Optimising - To what extent can modelling reduce the need for monitoring and enhance the information provided by monitoring?
4Supporting Change - How might a DNO implement technologies to support the transition to a Low Carbon Economy?
5Supporting Change - Which commercial models attract which customers and how will they be delivered?
Through the deployment of energy storage and power electronics the project will specifically explore questions centred on learning outcome four, these are:
4.1How could distributed solutions be configured into the DNO environment?
4.4How would network storage be used in conjunction with demand response?
Energy Management Trials
In the 2011 project bid submission NTVV identified a number of applications for energy storage and power electronics that explore their use as ‘network side’ solutions to enable more effective use of the existing network in accommodating the transition to a low carbon economy. Further details and extracts of the project bid submission are contained in Appendix 1.
Successful Delivery Reward Criteria
This report is written to develop and clarify the trials of energy storage and power electronics in the New Thames Valley Vision. In compiling this discussion document, the project is fulfilling Successful Delivery Reward Criteria (SDRC) 9.4a
CRITERION:Develop problem statement, hypothesis and test deployment programme for coordinated energy storage and power electronics on the Low Voltage distribution network - building on previous and current battery installation tests
EVIDENCE:Produce discussion document and deployment plan for energy storage and power electronics – include an assessment of the management of network losses and power quality
2.Problem Statement
Changing customer requirements
Network demand will change as individuals, small businesses and larger companies act either on theirconscience or in response to economic stimuli, to reduce their carbon footprint. The action customers takewill have many forms including: energy efficiency measures; the installation of solar thermal or photovoltaic(PV) panels and other small-scale renewable energy devices; and an increased uptake of electric vehicles. The New Thames Valley Vision looks to support changes in our customers’ energy behaviour as they move towards low carbon technology.
DECC's UK Low Carbon Transition Plan portrays a number of possiblepaths for the evolution of energy use as the low carbon economy advances. The document considers theimpact of a range of potentially disruptive technologies capable of changing the scale and nature of energyflows on the network, including:
- Electric cars
- Supplier drive demand side management
- Heat pumps
- Micro Generation
- Electrification of heat
- Electrification of transportation
These, amongst other factors, will have the effect of disrupting the predictability of maximum demands andprofiles that have heuristically evolved over the last few decades. These demand profiles are the centre ofour existing planning methodologies yet within the next few years will be ofdiminished value as thenature of load flows become more dynamic.
Better understanding of changing requirements through NTVV
The NTVV is developing new tools and techniques to better understand the changing energy requirements of customers connected to the Low Voltage network. By aggregatingand statistically grouping the modelled profiles of individual customers on each feeder the project will develop feeder power flow profiles. These will be validated against monitoring data from this project and other LCNF projects (to theextent that this data is made available) to identify whether behavioural trends can be drawn.
The project will create an agent-based forecasting model to enable short, medium and long-term demandpredictions with envelopes of uncertainty. Itis anticipated that take up of low carbon technology will not be uniform across customers, and hence neitheracross LV networks, but more sporadic in clusters. The forecasts will be designed to account for this non-uniformity. A key feature of both the modelling and forecasting techniques under development is that they are expected to beapplicable to, and have sufficient resolution for DNO purposes, regardless of whether smart meter data isavailable from every household and small business in the country.
Technical Standards and Efficiency
Electricity network customers can expect a regular and reliable supply of electricity which is capable of meeting power requirements within defined characteristics. The design and operation of our low voltage network ensures the network remains within a number of technical standards concerning voltage and thermal capacity. Each of these criteria has an impact on design and operation and is impacted as network usage changes. Economic and moral drivers dictate that networks should operate efficiently, where efficiency seeks to maximise utilisation and minimise loses at the lowest overall cost. These standards and drivers are described in more detail in Appendix 2
Voltage
The low voltage network is built with fixed transformer tapping ratios at the supplying 11kV/LV distribution substation with dynamic voltage control at the 11kV busbars of a primary substation only. The dynamic control seeks to maintain all connected customers within an acceptable voltage range but does not attempt to manage voltage variations for periods shorter than 1 minute.
Networks are designed to manage voltage with respect to: regulation, harmonic distortion, balance and flicker. Traditional engineering approaches for addressing poor voltage performance seek to: isolate ‘dirty’ loads, reduce current flow and/or reduce network impedance. Under all three of these approaches the network is not utilised at full thermal capacity and the connection of new loads or generation may be delayed until additional network assets can be installed. Clearly the installation of new network assets is a costly, disruptive and carbon-intensive operation.
Thermal Capacity
Electrical assets have finite thermal capacities beyond which their insulation performance deteriorates - excessive heat will cause an asset to fail. In a low voltage poly-phase cable the thermal capacity is the combined effect of all phase and neutral conductor limits
The traditional engineering approach for addressing poor thermal performance seeks to: distribute demand/generation evenly across phases at construction or during operation, if possible; split-up heavily congested networks by introducing additional interconnection; or overlay sections of reduced capacity. As with the traditional methods for addressing voltage performance, the above approaches require network reconfiguration and/or new asset installation which can be costly, disruptive and carbon-intensive operation.
Utilisation
The load-factor for the average low voltage customer[1] utilises only 3% of service cable capacity throughout the course of the year. To scale this up to the distribution network implies that the wider network is similarly under-utilised whilst simultaneously close to capacity in terms of instantaneous thermal and voltage limits
Since the traditional engineering approach for utilisation is not able to store energy at a local level, there is no scope to improve the utilisation of the network. With increased deployment of low carbon technologies, the network may be required to deal with even greater peaks for which the only traditional solution to maintain technical performance would be the creation of extra capacity and further reduction in utilisation
Loses
The technical[2] losses of a distribution network are a function of current flow through shunt and series impedances. Series losses are result in ‘real’ power lost from the system and constitute the largest contributor, these losses increase in proportion to the square of current flow. Shunt losses are entirely reactive but affect network performance by causing increased current flow and impacting voltage regulation. Analysis of the losses in a typical SSEPD GSP network[3] identified that 2.4% of the energy supplied was lost in local low voltage distribution
The traditional approach to technical loss reduction seeks to reduce network impedance through the installation of additional capacity and by attempting to balance connections across all phases. Both of these options are costly and disruptive. A proportionately greater improvement could be achieved by reducing peak current flow – however since traditional networks cannot store energy, this would be entirely at the discretion of the customer.
3.Solution
Power Electronics and Energy Storage
The New Thames Valley Vision will deploy power electronics and storage to allow the existing network to respond more flexibly. This will maintain the network within the technical standards identified above whilst also maintaining or improving the efficiency of the network as customers change their energy behaviour as they take up low carbon technologies.
Applications
The following applications will be trialled to support the technical requirements (identified above). In this table, the anticipated benefit is marked as ‘high’ where a direct improvement is expected and ‘medium’ where an indirect or consequential improvement is likely.
Voltage / Thermal / Efficiency / CI/CML and Emergency responseRegulation / Harmonic Distortion / Balance / Flicker / Phase / Neutral / Utilisation / Losses
1. Balancing load between phases (without storage)
Power electronics configured to a common DC busbar to allow dynamic redistribution of current from one phase to another to either equalise loading or manage voltages. This would result in reduced current flow peaks on the most loaded phase and address thermal constraints but would also have consequential improvements to efficiency and voltage regulation. / M / M / H / H / M / M
2. Storage to balance peaks and troughs
Application of storage and forecast energy consumption to optimise network utilisation within a specific capacity. Would have an impact on the directly connected LV circuit and also, in combination with other units on local substation and associated HV circuits. Direct benefits to thermal and efficiency measure with an indirect improvement in voltage regulation. / M / H / H / H / H
3. Balancing load between phases (with storage)
Combination of (1) and (2) above to allow optimisation across phases with additional capacity as enabled by spreading peaks and troughs across time. / M / M / H / H / H / H
4. Reactive voltage support (without storage)
Use of power electronics to modify current waveforms to adjust reactive component of load. Would result in increase to overall demand on circuit either from the same phase or adjacent phase. Reactive component use to counteract reactive volt-drop, mostly as a result of distribution transformer impedance. / H / H / M
5. Reactive voltage support (with storage)
As above, with the ability to use stored energy to alter reactive component without increasing overall instantaneous current on the basis that the energy store can be charged at some other convenient time. / H / H / M
6. Improve power quality & harmonics
Use of power electronics to act as active harmonic filters to identify and generate mitigating currents to improve power quality and harmonic limits. / H / H
7. Demand reduction
Use of reserve energy storage capacity to reduce demand during planned or fault outage to enable continuity of supply. / H
8. Frequency response
Use of reserve energy storage capacity to react to over or under frequency events by absorbing or releasing power. This would support the system operator’s duty to maintain frequency and would help customer maintain electricity during national frequency events / H
Traditional Network Reinforcement
As identified above, the traditional approach to maintaining the technical standards for voltage and thermal limits results in physical interventions to either increase capacity or reconfigure connections, where possible. Whilst these solutions remain valid, they do not necessarily encourage good network efficiency and are disruptive, slow to instigate and have a significant carbon impact.
To benchmark and asses the effectiveness of power electronics with energy storage, the NTVV will consider the implications of the main alternative, which is to do nothing new.
As the 2011 project bid submission identified, the most challenging part of replacing distribution assets would be at low voltage level, with the upheaval and cost of replacing individual service cables, substations and associated plant. The replacement value for the complete renewal of these assets just for SSEPD's two licences would be circa £3 billion.
Operational Management
Other than reinforcement, networks can seek to apply operational measures to manage changing network performance. It typically takes around 3 to 4 months from recognition of the need toreinforce an LV feeder to completing that reinforcement. During this time customers' generation is likely tobe tripping out on G83 protection (if there is an excess of generation connected) or LV network fuses arelikely to blow (if there is an excess of HP or EV connected). This would not be remedied by the deploymentof automatic replacement fuses such as the Bidoyng.
Increasing Requirements for Customer Equipment Performance
All equipment which connects to the Distribution Network must meet certain standards such that it does not unreasonably affect other customers connected to that network. This may be of particular relevance to harmonic performance. As an alternative to network reinforcement there may be relative merits in insisting on higher harmonic standards from appliances installed by customer.
Benchmarking Costs
From SSET 1008 LV Connected Batteriesproject, we have learnt that Energy Storage and Power Electronics can be expensive assets to deploy. Whilst we expect the cost of units to decrease as the technology matures and as greater economies of scale are realised, at present it does not seem feasible that units designed to address just one technical requirement would justify their deployment.
However, raw cost aside, Energy Storage and Power Electronics demonstrate the potential to exceed the performance of traditional reinforcement with respect to speed of deployment, level of disruption, coordinated improvements in technical standards and improved network efficiency.
The costs of energy storage and power electronics are broadly split as follows:
- Energy storage 80% of the cost
- Power electronics 20% of the cost
Therefore the challenge in driving economy is to combine multiple functionality into a single unit such that the same installed assets are able to simultaneously address a variety of technical standards. Likewise, improvement which can be realised using the electronics alone should be explored and delivered in such a way that allows storage to be added as and when economic.
4.Hypothesis
From a consideration of the Problem Statement in section 2 and the Solutions identified in section 3 as supported by the findings to date in SSET 1008 LV Connected Batteries project has drawn the following hypothesis:
Economic and flexible support for LV networks will be provided by power electronics with energy storage running smart control algorithms which make use of forecasted demand to provide a coordinated response to addresses the technical standards of voltage and thermal performance in the most efficient manner possible.
5.Test Deployment Programme
To assess the hypothesis of section 4the NTVV has developed the test plan detailed in this document. The following summary illustrates the linkages between the hypothesis and the test plan.
Summary
Hypothesis statement / Test Plan“… for LV networks…” /
Location
The fundamental aim of NTVV is to better understand and anticipate customer behaviour, which will help toreduce the uncertainty about future demands on distribution networks. The Thames Valley is considered to be an ideal location for such a project due to the 'ordinariness' of the network. The distribution system has nounique features; is of average age and reliability; has no significant low carbon initiatives in the areaand no eco-towns. In short, it is typical of much of the UK. We believe therefore that the findings from NTVVwill be applicable to much of the country and thus the learning useful to all DNOs.“Economic and flexible support…”
“… most efficient manner possible… /
Assessment criteria
Distribution networks are built to serve the needs of their customers and as customers move to lower carbon technologies, the networks will need to adapt so that the same reliable levels of service can be maintained. Through the assessment of Energy Storage and Power Electronics alongside traditional reinforcement the NTVV will assess the comparative benefits of:- Cost
- Speed of deployment
- Level of disruption
- Impact on technical standards (per standard and in combination)
- Impact on efficiency
“… the technical standards of voltage and thermal performance…”
“… coordinated response…” /
Network impact
The project will trial energy storage and power electronics units with the ability to operate the flowing functions in combination:1. Balancing load between phases (without storage)
2. Storage to balance peaks and troughs
3. Balancing load between phases (with storage)
4. Reactive voltage support (without storage)
5. Reactive voltage support (with storage)
6. Improve power quality & harmonics
7. Demand reduction
8. Frequency response
As identified in section 3 these functions will provide varying degrees of support to:
- Voltage – regulation, harmonic distortion, balance and flicker
- Thermal Limits – phase and neutral
- Efficiency – Utilisation and Losses
- CI/CML and Emergency response
“… smart control algorithms which make use of forecasted demand…”
“… coordinated response…” /
Control environment
The NTVV is developing algorithms to analysis and forecasts future energy consumption on the low voltage network in Bracknell. The Energy Storage and Power Electronics test plan will take advantage of this data and will assess its application in the practical control of field dispatched units.To facilitate this, the NTVV will deploy battery units which pass all relevant metrology from power electronics and energy storage along with other network information back to central system. The central control scheme then simulate various control level of control authority including ‘simulation’ of local control within the unit and also agent based communication between units
Staged DeploymentPlan